In a mobile radio communications system, information is exchanged between a mobile unit and a base station by radio signals. Since each base station can communicate with a mobile unit only within its radio coverage (cell), a plurality of base stations are needed to cover the whole service area. To avoid interruption of service when the mobile unit moves from one cell to another cell (roaming), a method is required to connect the mobile unit with the second cell at an appropriate time. The process of transferring between stations is called “handover.”
Methods of implementing handover of a call to or from a mobile unit between cells in a mobile radio communication system affect the overall efficiency and quality of the service. Therefore, it is important to provide a method of achieving a fast and reliable handover between cells.
Generally, the speed of mobile unit is an important factor to determine whether it is worthwhile implementing handover of a mobile unit between cells. The vehicular speed can be measured directly, but the method is not suitable for cellular radio applications. As an alternative, the vehicular speed may be estimated by monitoring the Doppler shift of a carrier frequency. FIG. 1 shows the conventional method of estimating movement speed of a mobile unit. First, a signal from the mobile unit is obtained in step 101. An envelope of the signal is calculated and the envelope is squared in step 103. According to the result of step 103, a correlation coefficient is calculated in step 105. Then, a corresponding Doppler frequency is obtained in step 107 by referring to a corresponding relationship between correlation coefficient and Doppler frequency. Lastly, the movement moving speed of the mobile unit is estimated according to the Doppler frequency in step 109.
However, this conventional method was limited. It can not accurately measure the movement speed of the mobile unit when the mobile unit moves at a low velocity. FIG. 2 shows the performance of the prior art. Three distinct curves A, B, and C respectively represent three mobile units having distinct powers of carrier signal. Obviously, when the three mobile units move below a specific speed, e.g. 18 km/hr, the curves A, B, and C bend down sharply. As such, one correlation coefficient mean value may map two distinct speeds, which prevents the system from determining the correct movement speed of the mobile unit.